Using the wrong type of fertilizer
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All-purpose fertilizers work for most plants, but some need specific nutrients or a more specialized balance for the best growth. This can lead to nutrient deficiencies over the life of your plant. For example, different manures and composted material that might have a lot of macro elements (Nitrogen, Phosphorous, Potassium) but might lack sufficient levels of micro elements. You won't notice an imbalance right away but over time this can have an effect on the growth and development of your plant.
SIGNS OF USING AN IMBALANCED NUTRIENT SOURCE
Weak growth
Discoloration of the foliage - different deficiencies will present themselves differently. The leaf might look pale, got green veins on a pale base, or get reddish or purple pigmentation.
Note that the above signs can also be caused by other issues with your plant.
WHAT TO DO NOW
CARNIVOROUS PLANTS
Carnivorous plants should not be watered with fertilizer. Most species are adapted to grow on very nutrient and mineral poor soils and can be seriously damaged if watered with fertilizer or even when planted in nutrient-rich soil. Instead you'll have to feed them in other ways.
For species producing pitchers (Nepenthes, Heliamphora, Sarracenia, Cephalotus) you can use diluted liquid fertilizer to pour into their pitchers, you can also use aquarium fish food flakes, dried bloodworms and other nutrient sources like bugs for them. The latter alternatives also work for other carnivorous plants (Dionaea, Drosera, Pinguicula, Utricularia etc.).
If you've accidentally watered your carnivorous plant with fertilizer, quickly flush the soil with water to remove it. If the plant is already showing signs of damage, it's best to repot entirely.
ERICACEOUS PLANTS
If ericaceous plants are not given specific fertilizer adapted to their lower pH soil, the plant will most likely exhibit deficiencies of certain nutrients. In many cases it's a lack of micro elements. Treating this is most easily done by switching to a fertilizer meant for low pH-soils.
HOW TO PICK THE RIGHT FERTILIZER - A SHORT GUIDE
When choosing the type of fertilizer to use for your houseplants, the most important thing to keep in mind is the N-P-K ratio (Nitrogen- Phosphorus - Potassium) as these are the key nutrients that your plant needs. Where this ratio is a higher value, it means that the fertilizer is more concentrated, and therefore less is needed. However, these ratios should be well-balanced. Some fertilizers aso contain additional micronutrients - these are called ‘complete’ fertilizers. It’s important to choose a fertilizer that is formulated for houseplants specifically - it should state this on the packaging - as these might have different N-P-K ratios to garden fertilizers. If you use a fertilizer that is not intended for houseplants, it can be too strong and you risk harming your plants.
Planta recommends using liquid fertilizer, as it is easy to use and measure, and generally priced affordably.
You can choose between using liquid fertilizers of organic or mineral origin. Organic ones are usually derived from composted plant matter or manure, while mineral fertilizers are chemically produced. Both alternatives work really well for the plant to grow as it should but both can also lack sufficient micro elements depending on the product.